Thermostat

ABSTRACT

A thermostat including a nichrome wire anticipator. The wire is bent into an arc and has a spring to it. The wire, bare of insulation, is placed in contact with the bare bimetal. The spring character of the wire holds it in pressure contact with the bimetal. The wire is everywhere insulated from the bimetal except over a relatively short length of the wire near the midpoint along its length for maximum heat conduction with the bimetal shorting out a substantial length of the wire.

United States Patent [72] inventor Roland D. Beck La Crescenta, Calif. [21] Appl. No. 32,304 [22] Filed Apr. 27, 1970 [45] Patented Nov. 16, 1971 [73] Assignee International Telephone and Telegraph Corporation New York, NY.

[54] THERMOSTAT 5 Claims, 5 Drawing Figs.

52 u.s. Cl 337/100, 337/107. 337/377 [5 I] Int. Cl ..ll0lh37/34, l-lOih 61/013 [50] Field oiSearch 2i9/5li; 337/99, 100, l02, 105, I07, 377, 378

[56] References Cited UNITED STATES PATENTS 3,3 i2,802 4/1967 Ryckman, Jr 337/102 3,2l L863 Levinn 337/107 2,920, i 65 l/i960 Dittman et al. 337/107 2,905,790 9/1959 Markham 337/i07 X 2,23 l,] 15 2/l94l Garrey 337/99 X 3,339,043 8/1967 Baak 337/100 X Primary ExaminerBernard A. Gilheany Assistant Examiner- Dewitt M. Morgan Attorneys C. Cornell Remsen,.lr., Walter J. Baum Paul W. Hemminger, Charles L. Johnson,l r and Thomas E. Kristofferson THERMOSTAT BACKGROUND OF THE INVENTION This invention relates to heat-sensitive elements and, more particularly, to a thermostat having a highly efiicient heater for use as an anticipator.

A prior art thennostat for use in turning a solenoid gas valve on and off in a conventional gas furnace for use in homes and offices and the like normally includes a bimetal spiral to operate a switch for energizing the valve solenoid. However, when the furnace is turned on by the switch, the furnace will tend to overshoot. That is, the room will get too hot before the thermostat turns the furnace off. For this reason, an anticipator is conventionally used to heat the bimetal so that the thermostat will open the switch in advance of the time it would otherwise shut off the furnace. Such an anticipator is simply a resistive winding which receives electric current as long as the switch is closed. For example, see U.S. Pat. No. 3,339,043.

Notwithstanding the advantage of an anticipator, its use causes another problem. As the heating load increases, the thermostat cycles more often. This causes the anticipator to pump more heat into the thermostat. As a result, the average bimetal temperature is raised further above ambient, and the thermostat fails to switch on the furnace until ambient has fallen below a comfortable temperature. This failure is called droop. Moreover, for a fairly constant ambient temperature, a large cycle rate is desired. However, an increase in cycle rate increases droop.

Prior art thermostats have difficulty maintaining a better cycle rate than 3 cycles per hour if the droop is 40 F. or less. This performance is poor because a cycle rate of 6 cycles per hour is desirable. However, with such a cycle rate, the droop increases to an intolerable 80 F.

SUMMARY OF THE INVENTION In accordance with the device of the present invention, the above-described and other disadvantages of the prior art are overcome by providing an anticipator in contact with the heatsensitive element of a thermostat.

Another feature of the invention resides in electrically insulating most of the element from the anticipator.

According to a special feature of the invention, the anticipator is a single spring wire resistor made of nichrome. The wire is nearly straight. The bimetal of the thermostat is also bare. The spring wire is held, by the spring thereof, into pressure contactwith the bimetal over a small area on both. Heat conduction from wire to bimetal is strikingly improved by a metalto-metal contact and by the spring action. A cycle rate of 6 cycles per hour with a droop of 2 F. is thereby made possible. The present invention thus has a performance much improved over that of the prior art. Note will be taken that the present invention makes possible a high cycling rate with little droop because the little wire efficiently heats the bimetal. Less power input to the wire is required because it is an efficient heater. Since less heat is put into the thermostat, droop is less.

The above-described and other advantages of the invention will be better understood from the following description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which are to be regarded as merely illustrative:

FIG. 1 is a side elevational view of the thermostat of the present invention;

FIG. 2 is an end elevational view thereof;

FIG. 3 is a side elevational view with the bimetal removed;

FIG. 4 is an end elevational view of the anticipator wire; and

FIG. 5 is a schematic diagram of the circuit of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the drawings in FIG. 1, a thermostat bimetal is indicated at 10 having an electrical switch contact 11. Contact 11 is fixed to bimetal 10 by an electrical insulator 12. A fixed contact 13 is mounted on an insulator l4. Bimetal 10 is fixed to a shaft 15 rotatable through a housing I6 with a knob 17. Shaft 15 and knob 17 are conventional. Shaft 15 is maintained in a fixed axial but rotatable angular position through housing 16 by conventional means. See FIG. 2 for bimetal 10, shaft 15, anticipator 18, housing 16, and knob 17.

In FIG. 1, the location of the anticipator is indicated generally at the dotted line 18.

As shown in FIG. 2, anticipator I8 is a nichrome wire having a circular cross section. The wire 18 lies generally in plane at an angle A relative to housing 16. In the said plane, wire 18 is bowed as indicated in FIG. 4.

As shown in FIG. 3, wire 18 has a location relative to bimetal 10 as indicated by the dotted line. Circuit terminals 19 and 20 are fixed to housing 16, which is made of an insulator. The ends of wire 18 are electrically and mechanically fixed to terminals 19 and 20. Wire 18 is resilient and has a spring to it. It therefore is somewhat rigid in the position shown in FIG. 3. As shown in FIG. 2, it is bare wire and by its own spring action is forced into pressure contact with the bare metal of bimetal 10.

The circuit of the invention is shown in FIG. 5. A transformer 2] has a primary winding 22 and a secondary winding 23. Secondary 23, anticipator 18, contacts 11 and 13, and winding 24 of the solenoid gas valve for a gas furnace are connected in series circuit 25. A variable resistor 26 is connected in parallel with anticipator 1-8.

Resistor 26 is set at installation to give a fixed current through anticipator 18 when switch 27 is closed. This current would vary with the type of winding 24. Resistor 26 is adjusted depending upon the amount of current winding 24 takes.

In operation, anticipator 18 is heated each time winding 24 is energized to turn the gas furnace on by the periodic closure of contacts 11 and 13.

The anticipator 18 has considerably less droop than prior art anticipators because wire 18 has a metal-to-metal contact with the metal of bimetal 10. Efficient heating of bimetal 10 requires less power and reduces droop. The efiiciency is improved by the spring contact of wire I8 with bimetal I0.

It is also a feature of the invention that bimetal 10 is insulated from contacts 11 and 13 to prevent a substantial length of wire 18 from being shorted out.

Note'will be taken that only a short length of wire 18 contacts bimetal 10 near the top of the bow shown in FIG. 4.

Anticipator 18 may have a resistance of about 0.5 ohm. Resistor 26 may have a resistance of about 2.5 ohms.

Wire 18 may be about 0.007 inch in diameter and about l.0 inch long.

What is claimed is:

l. A thermostat comprising a heat-sensitive element; an electrical conductor positioned in contact with said element, said conductor having at least one surface portion bare of insulation in contact with said element, said element and said conductor being made of metal, said conductor and said ele ment being free of insulation at least at a point of mutual contact to permit efficient heat transfer therebetween by conduction; a power circuit to pass electric current through said conductor to cause it to heat, said conductor being made of spring metal, said conductor being bowed, the spring thereof holding it in pressure contact with said element.

2. A thermostat comprising: a heat-sensitive element; an electrical conductor positioned in contact with said element,

said conductor having at least one surface portion bare of in-.

ductor to cause it to heat, said conductor being positioned in contact with said element over a relatively small length thereof as compared to its total length so that the metal of said element will not short out a considerable portion of the length of said conductor, said element being everywhere insulated from said power circuit except by contact with the conductor over said relatively small length thereof.

3. A thermostat comprising: a heat-sensitive element; an electrical conductor positioned in contact with said element,

said conductor having at least one surface portion bare of insulation in contact with said element, said element and said conductor being made of metal, said conductor and said element being free of insulation at least at a point of mutual contact to permit efi'icient heat transfer therebetween by conduction; a power circuit to pass electric current through said conductor to cause it to heat, said element being a bimetal strip wrapped in a spiral; and means to rotate said bimetal strip about the axis of said spiral, said conductor contacting said spiral at a distance from said axis equal to about one-half the radius of the largest spiral turn.

4. The invention as defined in claim 3, wherein said conductor is resilient, said conductor being shaped to form a spring andpositioned in a manner such that the spring action holds the conductor and element in pressure contact with each other.

5. The invention as defined in claim 3, wherein said element is a strip of two dissimilar metals bonded together to cause movement thereof when said strip is heated or cooled, a

switch, said movement being adapted to open and to close said switch, an electrical power supply, a fuel valve having a solenoid winding, and a variable resistor connected in parallel with said conductor, a series circuit including said conductor, said switch, said winding and said power supply all connected in series, said conductor having a circular cross section, means to fix the ends of said conductor rigidly to said circuit, said conductor being made of a nichrome wire spring 0.007 inch in diameter, said wire being bowed, said bow lying in compression against said strip, the spring character of said wire forcing it in pressure contact with said strip, said strip being located sufficiently close to said wire to deform the bow thereof slightly to cause a contact pressure to exist between said wire and said strip, said wire being bare of insulation, said strip also being bare of insulation, the bare metal of said wire lying in contact with the bare metal of said strip, said strip being spaced from said circuit except by the said pressure contact of said wire thereagainst, said strip being wound in a spiral, said wire contacting said spiral at the edge of a least one turn thereof at a position approximately halfway between the spiral center and the radius of the outside tum thereof, said resistor having a resistance of about 2% ohms, said wire having a resistance of about one-half ohm, said wire being about 1 inch long, the outside turn diameter of said spiral being less than 1 inch, said wire extending generally in a diametral direction across said spiral touching the same at about the midpoint of said wire. 

1. A thermostat comprising a heat-sensitive element; an electrical conductor positioned in contact with said element, said conductor having at least one surface portion bare of insulation in contact with said element, said element and said conductor being made of metal, said conductor and said element being free of insulation at least at a point of mutual contact to permit efficient heat transfer therebetween by conduction; a power circuit to pass electric current through said conductor to cause it to heat, said conductor being made of spring metal, said conductor being bowed, the spring thereof holding it in pressure contact with said element.
 2. A thermostat comprising: a heat-sensitive element; an electrical conductor positioned in contact with said element, said conductor having at least one surface portion bare of insulation in contact with said element, said element and said conductor being made of metal, said conductor and said element being free of insulation at least at a point of mutual contact to permit efficient heat transfer therebetween by conduction; a power circuit to pass electric current through said conductor to cause it to heat, said conductor being positioned in contact with said element over a relatively small length thereof as compared to its total length so that the metal of said element will not short out a considerable portion of the length of said conductor, said element being everywhere insulated from said power circuit except by contact with the conductor over said relatively small length thereof.
 3. A thermostat comprising: a heat-sensitive element; an electrical conductor positioned in contact with said element, said conductor having at least one surface portion bare of insulation in contact with said element, said element and said conductor being made of metal, said conductor and said element being free of insulation at least at a point of mutual contact to permit efficient heat transfer therebetween by conduction; a power circuit to pass electric current through said conductor to cause it to heat, said element being a bimetal strip wrapped in a spiral; and means to rotate said bimetal strip about the axis of said spiral, said conductor contacting said spiral at a distance from said axis equal to about one-half the radius of the largest spiral turn.
 4. The invention as defined in claim 3, wherein said conductor is resilient, said conductor being shaped to form a spring and positioned in a manner such that the spring action holds the conductor and element in pressure contact with each other.
 5. The invention as defined in claim 3, wherein said element is a strip of two dissimilar metals bonded together to cause movement thereof when said strip is heated or cooled, a switch, said movement being adapted to open and to close said switch, an electrical power supply, a fuel valve having a solenoid winding, and a variable resistor connected in parallel with said conductor, a series circuit including said conductor, said switch, said winding and said power supply all connected in series, said conductor having a circular cross section, means to fix the ends of said conductor rigidly to said circuit, said conductor being made of a nichrome wire spring 0.007 inch in diameter, said wire being bowed, said bow lying in compression against said strip, the spring character of said wire forcing it in pressure contact with said strip, said strip being located sufficiently close to said wire to deform the bow thereof slightly to cause a contact pressure to exist between said wire and said strip, said wire being bare of insulation, said strip also being bare of insulation, the bare metal of said wire lying in contact with the bare metal of said strip, said strip being spaced from said circuit except by the said pressure contact of said wire thereagainst, said strip being wound in a spiral, said wire contacting said spiral at the edge of a least one turn thereof at a position approximately halfway between the spirAl center and the radius of the outside turn thereof, said resistor having a resistance of about 2 1/2 ohms, said wire having a resistance of about one-half ohm, said wire being about 1 inch long, the outside turn diameter of said spiral being less than 1 inch, said wire extending generally in a diametral direction across said spiral touching the same at about the midpoint of said wire. 